**3.3 Microelement content of digestate**

Plants, animals and humans require trace amounts of some heavy metals like copper (Cu), zinc (Zn), while others like cadmium (Cd), chromium (Cr), mercury (Hg), lead (Pb) are toxic for them. Heavy metal content of the feedstock usually originates from anthropogenic source and is not degraded during AD. The main origins of the heavy metals are animal feed additives, food processing industry, flotation sludge, fat residues and domestic sewage.

With a N load of 150 kg ha-1, the heavy metals load into the soil (Cd, Cr, Cu, Ni, Pb, Zn) were lower in the case of digestate addition comparing to the compost and sewage sludge treatments while were higher in some heavy metals (Cu, Ni, Pb, Zn) comparing to the mineral fertilizer (Pfundtner, 2002).

### **3.4 Organic matter content of digestate**

The amounts of organic dry matter and the carbon content of digestate are decreased by the decomposition of easily degradable carbon compounds in the digestors (Stinner et al., 2008). Menardo et al. (2001) found the degree of organic matter (OM) degradation between 11.1%

et al., 2008). The higher N content of a digestate comparing to the composts is the consequence of the N concentration effect because of carbon degradation to CO2 and CH4

The NH4 content of the digestate is about 60-80% of its total N content, but Furukawa and Hasegawa (2006) reported 99% of NH4-N of the digestate originated from kitchen food wastes. Generally, the NH4-N concentration is increased by the protein-reach feedstock (Kryvoruchko et al., 2009) like diary by-products and slaughterhouse waste (Menardo et al., 2011). The conversion of organic N to NH4-N allows its immediate utilization by crops (Hobson and Wheatley*,* 1992). The higher amount of NH4-N and the higher pH predominate over the factors (lower viscosity, lower dry matter content) which could reduce the ammonia volatilization from the digestate (Möller & Stinner, 2009). The emission of ammonia could be decreased by different injection techniques which lower the air velocity above the digestate and because of the bound of gaseous ammonia to soil colloids and soil water (McDowell and Smith, 1958). The application depth has a significant effect on ammonia volatilization. Surface application of a liquid biofertiliser caused the loss of 20-35% of the applied total ammoniacal N while disc coulter injection into 5-7 cm depth reduced the ammoniacal loss to 2-3% (Nyord et al, 2008). This method should be used also in the case of

Digestate has higher phosphorus (P) and potassium (K) concentration than that of composts (Tambone et al., 2010) therefore it is more suitable for supplement of these missing macronutrients in soils. Furthermore, Börjesson and Berglund (2007) assumed all phosphorus in the digestate to be in available forms, therefore digestate seems to be a useful material for supplement missing nutrients of soil, especially of the P and K. The average phosphorus-potassium ratio of digestates is about 1:3 which is excellent for grain and rape. Accumulation of P and K in soil could be avoided by the reduction of the applied digestate dose but in this case, for the supplement of nitrogen gap, the artificial fertilizer has to

Plants, animals and humans require trace amounts of some heavy metals like copper (Cu), zinc (Zn), while others like cadmium (Cd), chromium (Cr), mercury (Hg), lead (Pb) are toxic for them. Heavy metal content of the feedstock usually originates from anthropogenic source and is not degraded during AD. The main origins of the heavy metals are animal feed additives, food processing industry, flotation sludge, fat residues and domestic sewage. With a N load of 150 kg ha-1, the heavy metals load into the soil (Cd, Cr, Cu, Ni, Pb, Zn) were lower in the case of digestate addition comparing to the compost and sewage sludge treatments while were higher in some heavy metals (Cu, Ni, Pb, Zn) comparing to the

The amounts of organic dry matter and the carbon content of digestate are decreased by the decomposition of easily degradable carbon compounds in the digestors (Stinner et al., 2008). Menardo et al. (2001) found the degree of organic matter (OM) degradation between 11.1%

and N preservation during AD (Tambone et al., 2009).

digestate application to reduce ammonia volatilization.

**3.3 Microelement content of digestate** 

mineral fertilizer (Pfundtner, 2002).

**3.4 Organic matter content of digestate** 

be used.

and 38.4% in the case of different ingestates, highest loading rates and hydraulic retention times while Marcato et al. (2008) found this value of 53%. If the organic loading rate of biogas plant is high and the hydraulic retention time is short, the digestate will contain a considerable amount of undigested OM, which is not economic and not results a good amendment material. However, the OM content of digestate is more recalcitrant and therefore the microbial degradation and soil oxygen consumption can be decreased by its application (Kirchmann & Bernal, 1997).

The adequacy of digestate as soil amendment is based on its modified OM content. Most OM is converted into biogas, while the biological stability of remaining OM was increased during AD with the increase of more recalcitrant molecules like lignin, cutin, humic acids, steroids, complex proteins. These aliphatic and aromatic molecules are possible humus precursors with high biological stability (Tambone et al., 2009). Pognani et al. (2009) found the increase of these macromolecules′ quantities in the course of AD as it can be seen in Table 3.


Table 3. Changes in macromolecules content on the course of AD *(Data from Pognani et al., 2009)*

Similarly, the rate of lignin-C, cellulose-C and hemicellulose-C are increased in the organic matter content after AD of cattle and pig dung (Kirchmann & Bernal, 1997). The increase of these macromelecules-C were 2.4-26.8 %, 14.2-13.9 % and 7.3 % in the manures, respectively. The hemicellulose-C content in the anaerobically treated pig dung was decreased by 23.8 %. However, the increase of non-decomposable carbon content of digestate is always smaller than that of composts (Gómez et al., 2007). On the other hand, improving the fertilizer effect of a digestate with its higher decomposable carbon content results in an increase in roots and crop residues which may have an important effect on the soil organic matter content.
